Conical air filter

ABSTRACT

An air duct system includes a conical fabric filter disposed within a cylindrical air duct. In some embodiments, both the filter and the air duct are inflatable. A fabric collar and a pair of zippers not only allow the filter to be readily removed for cleaning, but also allow the air duct system to continue operating with the filter removed. Pleats can provide the filter with more surface area, and the pleats can be interconnected in an alternating pattern to inhibit the filter from over inflating.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The subject invention generally pertains to air filters and morespecifically to a fabric filter for use inside an air duct.

2. Description of Related Art

Fans or blowers are used along with ductwork to circulate air through aroom or area of a building. The blower typically draws air from withinthe room through a return air duct and then forces the air back into theroom through a supply air duct. To heat or cool the air, the blower mayalso force or draw the air across a heat exchanger.

To help prevent dust from accumulating on the heat exchanger, blower,and ductwork, often a conventional filter is installed at the downstreamend of the return air duct. Finer, less porous filters are used wheredust removal is more critical, such as in so called clean rooms or inbuildings having occupants with dust-related allergies. Unfortunately,fine filters usually create a higher pressure drop that reduces theamount of airflow. To minimize the pressure drop, a filter's effectivecross-sectional area can be increased in various ways, such as by addingpleats to the filter, installing the filter at an angle relative to theduct, or by forming the filter as an elongated bag that extendslengthwise into an air duct.

Some examples of filters that are elongated along the direction ofairflow are disclosed in U.S. Pat. Nos. 2,853,154; 3,151,962; 3,195,296;3,204,391; 3,204,392; 3,396,517; and 3,538,686. When mounting suchfilters within a return air duct, upstream of the blower, a significantdistance is needed between the blower and where the filter attaches tothe duct, simply due to the length of the filter. In many cases, thiscan be difficult or impossible to do, because of bends or elbows in theductwork. Also, much of the ductwork is usually inaccessible, as it isoften installed within the walls of the building or between the floorand ceiling. So filters in a return air duct are typically installedimmediately adjacent the blower, which may prohibit the use of anelongated filter or at least significantly limit its length.

On the other hand, if an elongated air filter were installed in thesupply air duct, the filter would do little in preventing dust fromaccumulating on the blower and the heat exchanger, because dust oftenoriginates in the room. With a filter installed in the supply air duct,dust from the room could pass across the blower and heat exchangerbefore ever reaching the filter.

Moreover, if elongated filters of current designs were installed withina generally cylindrical duct having a pliable fabric wall, thenon-conical shape of the filter may cause the fabric of the duct toflutter, due to uneven patterns of airflow velocity. If thecross-sectional area of airflow between the exterior of an elongatedfilter and the interior of the cylindrical fabric duct is notcircumferentially uniform, as could be the case with a flat-sided filterwithin a cylindrical duct, localized areas of higher velocity may exist.Also, abrupt changes in velocity along the length of a fabric duct mayalso cause the fabric to flutter.

SUMMARY OF THE INVENTION

In some embodiments, an air duct system includes a conical filterdisposed within a cylindrical duct.

In some embodiments, an air duct system includes an inflatable conicalfilter with pleats.

In some embodiments, the pleats are interconnected in an alternatingpattern of connection points to inhibit the filter from billowingexcessively outward.

In some embodiments, an air duct system includes a blower and a heatexchanger interposed between an upstream pre-filter and a downstreamconical filter, which is less porous.

In some embodiments, an inflatable fabric filter is disposed within aninflatable fabric air duct.

In some embodiments, the fabric wall of the air duct is air permeable.

In some embodiments, the integrity of a fabric air duct can bemaintained regardless of whether the elongated filter is attached to theduct.

In some embodiments, a zipper removably attaches an elongated filter toa fabric air duct.

In some embodiments, a plurality of conical filters have the same lengthto diameter ratio even though the filters are of different diameters forvarious diameter air ducts.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cutaway view of an air duct system with a fabric air ductand a conical fabric filter.

FIG. 2 is a cutaway view of an air duct system with a relatively rigidair duct and a conical fabric filter.

FIG. 3 is similar to FIG. 1, but with the fabric duct and filterdeflated.

FIG. 4 is a perspective view of the filter used in the air duct systemof FIG. 1.

FIG. 5 is a closer up view of the supply air duct and conical filter ofFIG. 1.

FIG. 6 is similar to FIG. 5, but with the filter removed and twosections of the supply air duct zipped together.

FIG. 7 is similar to FIG. 4, but showing a fabric conical filter that ispleated.

FIG. 8 is a cross-sectional view taken along line 8—8 of FIG. 7.

FIG. 9 shows one of a plurality of conical air filters.

FIG. 10 is similar to FIG. 9, but showing a larger filter with the samelength to diameter ratio.

DESCRIPTION OF THE PREFERRED EMBODIMENT

An air handling system 10 of FIG. 1 is used to heat or cool an area 12of a building 14. To do this, system 10 includes a blower 16; a heatexchanger 18; a pre-filter 20; a finer, less porous inflatable filter22; a supply air duct 24; and a return air duct 26. Heat exchanger 18 isschematically illustrated to represent any device for heating or coolingair, such as by electrical resistance or by heat transfer with anotherfluid, such as refrigerant, water, or glycol. A housing 28 can encloseone or more of the components of system 10.

In operation, blower 16 draws air 30 from area 12, through return airduct 26 and across pre-filter 20, with pre-filter 20 being anyconventional filter known to those skilled in the art. Pre-filter 20 canbe used to capture the larger dust particles in the air that mightotherwise accumulate on heat exchanger 18 and blower 16. Pre-filter 20also helps prevent large dust particles from quickly plugging up theless porous filter 22 in supply air duct 24.

After the air passes through pre-filter 20, blower 16 draws the airacross heat exchanger 18. Blower 16 then discharges the air throughinflatable filter 22, through supply air duct 24, and into area 12through the pores or other openings in supply duct 24. Filter 22, beingrelatively fine, can be used to remove smaller dust particles that wereable to pass through pre-filter 20. In some embodiments, the fabricmaterial of filter 22 is provided by 3M of St. Paul, Minn., and has astandard particle removal efficiency of 80 to 90%, at 150 to 300cfm/ft², with a static pressure drop of 0.2 inches of water.

Conical fabric filters, such as filter 22, can be installed withinvarious types of ducts. The supply air duct can be made of sheet metalor some other relatively rigid material, as is the case of conicalfilter 22′ in supply air duct 32 of FIG. 2, or can be made of a pliablefabric 34, as is the case of duct 24. With a metal air duct, airregisters 36 provide one or more openings for air to discharge into area12. As an alternative or in addition to registers 36, the fabric of airduct 24 may be air-permeable and/or be provided with cutouts ordischarge openings 38 that deliver air to area 12. Examples of fabricair duct 24 are disclosed in U.S. Pat. Nos. 5,655,963 and 5,769,708,which are specifically incorporated by reference herein.

In the example of FIG. 1, the fabric wall of duct 24 has a generallycylindrical or tubular shape when inflated by the discharge pressure ofblower 16. However, when the heating or cooling demand of area 12 hasbeen satisfied, blower 16 may turn off, which deflates filter 22 andleaves the fabric walls of duct 24 hanging relatively limp, as shown inFIG. 3. Some fabric air ducts have a rigid frame that helps hold thefabric walls of the duct in a generally tubular shape even when theblower is not running. Such frame-supported ducts are also well withinthe scope of the invention.

Filter 22 can be installed within an air duct (metal or fabric, supplyor return) in various ways. In a currently preferred embodiment, acollar 40, made of fabric or some other material, couples filter 22 to afirst segment 24 a and a second segment 24 b of fabric air duct 24.Referring further to FIG. 4, fabric rim 42 at a base 44 of filter 22 issewn or otherwise attached to the interior of collar 40. Collar 40includes two half-zippers 46 and 48 that removably interlock with matinghalf-zippers 50 and 52 on supply air duct 24, as shown in FIG. 5.Half-zippers 46 and 50 comprise a first zipper 54, and half-zippers 48and 52 comprise a second zipper 56. Zippers 54 and 56 allow filter 22 tobe temporarily removed from duct 24 for filter cleaning or replacement.If filter 22 is removed for an extended period, half-zippers 50 and 52may be zipped together to re-establish a continuous supply air duct, asshown in FIG. 6.

To minimize the pressure drop created by filter 22 and to extend theperiod between filter cleanings, filter 22 is elongated to provide alarge surface area though which the air may pass. This is accomplishedby having filter 22, when inflated, be of a generally conical shape(i.e., most of its contour or outer envelope fits the shape of a cone).In some embodiments, filter 22 is in the shape of a cone (i.e.,substantially all of its contour or outer envelope fits that of a cone).

To help prevent the fabric walls of duct 24 b from fluttering, thevelocity and flow direction of the air between the exterior of filter 22and the interior of duct 24 b is kept as smooth as reasonably possible.This can be achieved by installing a conical filter within a cylindricalduct to create an airflow path whose annular cross-sectional areaincreases gradually from an upstream to a downstream end of filter 22.

To provide a conical filter with more surface area, a filter 58 can havea pleated fabric wall, as shown in FIGS. 7 and 8. The pleats rungenerally lengthwise with each pleat being connected to its two adjacentpleats in an alternating pattern of discrete points. For example, acentral pleat 60 lies between a first pleat 62 and a second pleat 64.Central pleat 60 has a central peak 60′ that zigzags between an adjacentfirst peak 62′ and a second peak 64′ of pleats 62 and 64, respectively.Central peak 60′ is attached to first peak 62′ at points 66, 68 and 70.Central peak 60′ is also attached to second peak 64′ at points 72, 74and 76. The alternating pattern of connection points inhibits theblower's discharge air pressure from flattening the pleats and restrainsfilter 58 to a generally conical shape.

To provide a plurality of conical filters that provide the same flowrate for a given area of filter material regardless of the duct'sdiameter, each filter's length to diameter ratio is the same. Forexample, in FIG. 9, a filter 78 in a first duct 80 has a diameter 82 of24 inches, as measure along a base 84 of conical filter 78, and has alength 94 of 120 inches, as measured from a center 86 of base 84 to anapex 88 of filter 78. Similarly, in FIG. 10, a filter 90 in a largerduct 92 has a diameter 96 of 48 inches and a length 98 of 240 inches,whereby both filters 78 and 90 have a length to diameter ratio of five(120/24=5, and 240/48=5).

Although the invention is described with reference to a preferredembodiment, it should be appreciated by those skilled in the art thatvarious modifications are well within the scope of the invention.Therefore, the scope of the invention is to be determined by referenceto the claims that follow.

We claim:
 1. An air duct system for conveying and filtering a stream ofair, comprising: an air duct adapted to convey the stream of air, theair duct having a generally tubular shape and comprising a fabric wall;an inflatable filter having a generally conical shape when inflated andattached to the fabric wall such that substantially the entire stream ofair that passes through the air duct also passes through the inflatablefilter; a first zipper that couples the inflatable filter to the airduct; a second zipper that couples the inflatable filter to the airduct; and a removable collar attached to the inflatable filter andattached to the first zipper and the second zipper, wherein the air ductincludes an upstream duct and a downstream duct with the removablecollar interposed therebetween via the first zipper and the secondzipper.
 2. The air duct system of claim 1, wherein the upstream duct isadapted to be connected to the downstream duct if the inflatable filterwere disconnected from the air duct.
 3. An air duct system for conveyingand filtering a stream of air, comprising: an air duct adapted to conveythe stream of air, the air duct having a generally tubular shape andcomprising a fabric wall; an inflatable filter having a generallyconical shape when inflated and attached to the fabric wall such thatsubstantially the entire stream of air that passes through the air ductalso passes through the inflatable filter; wherein the inflatable filterincludes a plurality of pleats that are interconnected to limit theextent to which the inflatable filter may billow; and wherein theplurality of pleats includes a first pleat having a first peak, a secondpleat having a second peak, and a central pleat having a central peakand being interposed between the first pleat and the second pleat,wherein the central peak is attached to first peak and the second peakat a first point and a second point respectively, wherein the firstpoint is spaced apart from the second point.
 4. An air duct system forconveying and filtering a stream of air, comprising: an air duct adaptedto convey the stream of air, the air duct having a generally tubularshape and comprising a fabric wall; an inflatable filter having agenerally conical shape when inflated and attached to the fabric wallsuch that substantially the entire stream of air that passes through theair duct also passes through the inflatable filter; and a heat exchangerand a pre-filter, wherein the heat exchanger with respect to a flowdirection of the stream of air is downstream of the pre-filter andupstream of the inflatable filter.
 5. The air duct system of claim 4,wherein the pre-filter is more porous than the inflatable filter.
 6. Anair duct system for conveying and filtering a stream of air, comprising:an upstream duct adapted to convey the stream of air; a pliabledownstream duct adapted to convey the stream of air; a collar interposedbetween the upstream duct and the downstream duct; a filter connected tothe collar being adapted to file the stream of air, wherein the filteris generally conical; wherein the filter includes a plurality of pleatsthat are interconnected to link the extent to which the filter maybillow; wherein the plurality of pleats include a first pleat having afirst peak, a second pleat having a second peak, and a central pleathaving a central peak and being interposed between the first pleat andthe second pleat, wherein the central peak is attached to first peak andthe second peak at a first point and a second point respectively,wherein the first point is spaced apart from the second point.
 7. An airduct system for conveying and filtering a stream of air, comprising: anupstream duct adapted to convey the stream of air; a pliable downstreamduct adapted to convey the stream of air; a collar interposed betweenthe upstream duct and the downstream duct; a filter connected to thecollar and being adapted to filter the stream of air, wherein the filteris generally conical; a blower and a pre-filter, wherein the blower withrespect to a flow direction of the stream of air is downstream of thepre-filter and upstream of the filter; and a heat exchanger and apre-filter, wherein the heat exchanger with respect to a flow directionof the stream of air is downstream of the pre-filter and upstream of afilter.
 8. The air duct system of claim 7, wherein the pre-filter ismore porous than the filter.
 9. An air duct system for conveying andfiltering a stream of air moving from upstream to downstream,comprising: a pre-filter; a blower downstream of the pre-filter; aninflatable filter downstream of the blower; a first air duct downstreamof the inflatable filter; a first zipper that couples the inflatablefilter to the first air duct; a second air duct upstream of the firstair duct and downstream of the blower; a second zipper that couples theinflatable filter to the first air duct; and a removable collarinterposed between the first air duct and the second air duct and beingattached to the inflatable filter.
 10. An air duct system for conveyingand filtering a stream of air moving upstream to downstream, comprising:a pre-filter; a blower downstream of the pre-filter; an inflatablefilter downstream of the blower; a first air duct downstream of theinflatable filter, wherein the inflatable filter includes a plurality ofpleats that are interconnected to limit the extent to which theinflatable filter may billow; and wherein the plurality of pleatsincludes a first pleat having a first peak, a second pleat having asecond peak, and a central pleat having a central peak and beinginterposed between the first pleat and the second pleat, wherein thecentral peak is attached to first peak and the second peak at a firstpoint and a second point respectively, wherein the first point is spacedapart from the second point.